Deciles give an element a ranking, compared with the average for an area. For example, choosing deciles under the 'maps' option for Rainfall Maps will give a rainfall map showing whether rainfall is above average, average or below average for the time period and area chosen.

How to calculate deciles
To calculate deciles you first need to arrange your data, for example rainfall totals, in ascending order (from lowest to highest). Next, divide the ranked data set into ten equal parts (i.e. ten blocks of 10%). The first group (the lowest 10% of rainfall totals on record) would be in decile range one, the second group in decile range two, up to the highest 10% of rainfall totals being in decile range 10.

A rainfall total in decile range 10 would be in the top 10%, higher than (at least) 90% of previous monthly observations. The highest total on record is at the very top of decile range 10, while the lowest total is at the bottom of decile range 1.